FAST KINETICS OF UNPAIRED ELECTRONS IN PHOTOSYNTHETIC SYSTEMS

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Abstract
Intact cells of Rhodospirillum rubrum exhibit a monotonic increase in electron spin resonance (ESR) signal intensity which reaches half its asymptotic value in about 30 msec on illumination with white light. Similar kinetics are exhibited by the decay of the ESR signal when illumination ceases. The corresponding time constants are of the order of several hundred msec with intact cells of Euglena. Following either onset or cessation of illumination, R. rubrum exhibits a 2-3 msec. lag before the resultant change in ESR signal intensity begins. This indicates that the unpaired electrons associated with the ESR signal are not due to a light-excited state of the primary absorber in photosynthesis. One or more electron or energy transfer processes must intervene between the primary light absorption event and the generation of unpaired electrons in R. rubrum photosynthesis. The kinetic changes in ESR signal intensity ixhibited by R. rubrum on onset or cessation of illumination may be regulated by the rates of one or more processes which result in the decay of the unpaired electrons generated by light.